1. FIELD OF THE INVENTION:
This invention relates to a method of synthesizing nitrato alkyl oxetanes.
2. DESCRIPTION OF THE PRIOR ART:
Energetic polymers are of particular interest in providing energetic elastomers which are suitable for use in binder systems of high-energy compositions, such as propellants, explosives, and gasifiers. For example, high-energy polyethers are curable with isocyanates to form elastomers suitable for use as propellant binders. High energy polyethers and elastomers formed by curing the same have the further advantage of being miscible with high-energy nitroester plasticizers, such as nitroglycerine, butanetriol trinitrate and trimethylolethane trinitrate. A nitroester-plasticized polyether elastomer containing high-energy mer units can provide a binder system which is inherently very high in energy.
U.S. Pat. No. 4,483,978 ('978), issued to Manser on Nov. 20, 1984, the teachings of which are incorporated herein by reference, describes in detail azido alkyl oxetanes and polyethers and cured elastomers derived therefrom. '978 also discloses that other energetic substituents in oxetanes include nitro and nitrato groups. While azido derivatized oxetanes, such as azido methyl methyl oxetane (AMMO) and bis (azidomethyl) oxetane (BAMO), are of interest, they cannot provide the level of propellant performance that nitrato- and nitro-derivatized oxetanes, such as nitrato methyl methyl oxetane (NMMO) and bis (nitratomethyl) oxetane (BNMO), can provide. For instance, the specific impulse of a BAMO/AMMO system with an oxygen to fuel ratio of 1.053 to 1 is about 1.4 while the specific impulse of a BNMO/BAMO system with an oxygen to fuel ratio of 1.122 to 1 is about 1.8. Smoke propellant results can be even more dramatic in some cases, showing an impulse increase of up to six seconds when nitrato- and nitro-derivatized oxetanes are used instead of azido-derivatized oxetanes.
However, nitrato-derivatized oxetanes have not been used to any significant extent in binders for high-energy compositions because there does not exist a practical synthesis for such compounds. The accepted literature method of producing nitrato alkyl oxetanes, specifically 3,3-Bis (nitratomethyl) oxetane, by nitrating a reaction mixture of trisnitrato pentaerythritol via an equal mixture of 80% nitric acid and 80 % sulfuric acid. This is unacceptable for large scale production of nitrato-derivatized oxetanes because the method results in the opening of the oxetane ring. The ring must be subsequently closed with sodium ethoxide in order to obtain the product. Additionally, although the method is reproducible, it produces uncontrolled fume-off during the nitration step and near explosive conditions.
Direct nitration of the hydroxyl alkyl oxetanes would avoid this circuitous route, but as the oxetane ring structure is highly strained, ring opened products are thermodynamically favored and tend to result, particularly under the highly acidic conditions under which most nitrations are conducted.